Frequency shift signalling system



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ATTORNEY H. K. ROBIN 2,833,85T FREQUENCY SHIFT SIGNALLING SYSTEM Filed Feb. 6, 1956 12 Sheets-Sheet 2 May 5, w58 H. k, ROBIN 2,833,857

FREQUENCY SHIFT SIGNALLING SYSTEM Filed Feb. 6, 195e 12 sheets-sheet 3 BY zi [N Nok j i ATTosENEY May 6, 1958 H. K. ROBIN 2,833,857

FREQUENCY SHIFT SIGNALLING SYSTEM Filed Feb. 6, 1956 l2 Sheets-Sheet 4 /NvENTogz A TTORNE Y ay 6, MSS H. K. ROBIN EREQUENCY SHIFT SIGNALLING SYSTEM Filed Feb. 6, 1956 12 Sheets-Sheeil 5 AMPLIFIER TUNING FORK P WOW LL PFU T www LLM FFU .Pl F Pow 1 ULM FFC n .....n PPU 1 .loc LLR PFU Ulu.. .il- N /N VEN www A TTORNE Y May w58 H. K. ROBIN 2,833,857

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' ATTORNEY May 6, 1958 Filed Feb. 6, 1956 H. K. ROBIN FREQUENCY SHIFT SIGNALLING SYSTEM Sheets-Sheet 1,01 We PCI, r l l comm. PHnse mun MPU' ssamm +45 PHASE ausmalen -45 'PC2 L/a 591. 5(6').

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FREQUENCY SHIFT SIGNALLING SYSTEM Filed Feb. e, 195e 1'2 sheets-sheet a GR YSTHL 056ML NTO FLIPFLOP CIRCUIT Fup-Flop emol/1r FLIP-i109 CIRGUI 7 Fup-fz op cmcu/ r FLIP -FLOP CIRGUI 7 /N VEN TOR- ATTORNEY May 6, 195s Filed Feb. 6; 1955 H. K. ROBIN FREQUENCY SHIFT SIGNALLING SYSTEM .l2 Sheets-Sheet 9 Re nc, .l

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FREQUENCY SHIFT SIGNALLING SYSTEM 12 Sheets-Sheet lO PUSH-PULL AMPLIFIEA RPA VATTORNEY May 6, 195s H. K RQBIN 2,833,857

FREQUENCY SHIFT SIGNALLING SYSTEM L Filed Feb. 6, 1956 s 12 Sheets-Sheet 11 A TTORNE Y May 6, i958 H. K. ROBIN FREQUENCY SHIFT SIGNALLING SYSTEM 12 Sheet-s-Sheet 12 Filed Feb. 6. 1956 United States PatentO Claims-.priorit1,f, applicationiGreatiBriiaim Deeember, v1950 The invention of thisI application,k whichis a continuationdn-part of application Serial No. 259,637, filed. De.`

cember 3, 195l,.now` abandoned, relates to electriesig nailing.A

Intelligence is often transmittedY by means` ofa sut:- cession of. bursts of oscillations; which. may-be of equal or=unequal durations. For example in telegraphy'emp ,loying carrier frequency shift keyingmark. and space signals. are applied to, transmit` burstsof oscillations.l of two. different :frequencies respectively; The. dilerence between: the. frequencies of. the. oscillations representing the mark. andspace signalslrnay, for example, `be a few hundredcycles per second. The mark and space; signals transmitted inA suchsystems are given. durations; dependent upon. theintelligenceto be transmitted, butit uslusual to arrange that eachtransmitted. b ursthas a-.durationwhieh is. anintegral .multiple of-aL selected, timeA interval.

Other systems .have beenproposed in-which a..succes,f sion -of items of.V intelligence are, transmitted by transmite tingras.. amodulation of a. carrier. oscillation a. succes.- sion of regularly recurringbursts .o f..oscillations,. the.sey; eral bursts of; oscillationsbeingof equal durations and thefrequencies of the. bursts representative. ofdiierent items. of yintelligence :having different` frequencies.;

Ina receiver in' such systemsitiis necessaryto provide, for. d emodulation purposes, a .local oscillator of known frequency: For example in the casetof, carrierfrequencyvshiftlteying the radio frequency oscillationsrepresenting markand spiace. signals have. to be` c.onyerted.lin,to.. posi tive and. negative-going; impulses for application toa telegraph relay. These irnpulsesiare.v obtained. inkno.w.n receivers by b eatinga .locally-generated oscillationwith the .received oscillations .to produce beatsf of, two: dif.- ferent frequencies alternately representing the.. mark.; and spacesigllals. These. beats; may, for example, have; frequencies of .6.00. and 1600 c./s. Theseareseparatedby means ofsuitable low. frequency, filters and. the.Y outputs ofJ the filters are rectiied andapplied to .provide the Y negativevvv and positive-going impulses. previously. referred t0.

It isv well known that drift in thev frequency. o f the locally-generated. oscillations leads` to eitherdistortion., or unintelligibility in thedemodulated signals. For. example in thecase. of the example previously referred totemploying carrier .frequency shift keying, thetwo output oscillations may differ in frequency. by only 8.00.7c./s. lfthe radio. frequency oscillations from whiclrthe outputA oscillations are derived are.. of` the. order. of.,. say, 20 mc./s. the locallyfgenerated. oscillations.- are. of the same order. The frequency of thel local. oscillator must therefore. be maintained stable within a sniallnurnber of cyclesv inl() millionin order to provideisatisfactory results..

Elaborate and costlyl apparatushas been proposed to ensure frequency stability of. the-.local oscillator. to. the degree necessary to ensure satisfactory results.. The usual practice is to transmit along withY the bursts of oscillation a pilot carrier which is of low amplitude com- ICC paredt withy that.. ofligthe bursts. The pilot `carrier may thenzbe amplified andused inthe receiver to synchronise a;.local`,oscillator. As` thearnplitude, of the pilot carrier isslow, however, itoften happensv that. due. to fading the amplitude .of the;pilot carrier fallsb totsuchalow value that control over the frequencyfpof the local oscillator in. the receiver isf-lost; Drift iii-the` frequency of; the 1ocal-,oscillator thenoccursJesultngrinidistortion or even unintelligibility. y

One object. ofy the presenninyenton ist-.to Provide au improved electric. dsonline-system. for transmitting in.- telligence in the, form of`l.. bursts.v of oscillationsinwwhich the need for a pilotcarrien is.removed.`

According to the.A present invention, an electric sig- Helling-f. System .comprises a transmitter. adopted-.t0 transf mitgasa modulation; of a carrier-oscillatioiiv` a-succession of bursts of oscillations, different bursts commencing; at different ones ofa succession;oftregularlyt recurling instante spacedapart by. a time` periodgt, and-areceiver adapted to` receive thetrans .rnitted` oscillations. and in which, in operation, the.received.,osscillationsfareapplied through twov transmission channels or netwoijlcsgtowan output. .oirouitadonted to.- provider-au Output Whose; frequency at any instant is equal to theldiifere.esbetween the;frequencies,l of theoutputs l of. the, twog` networks at that instant, one of the channels or network 'ncluding moons.. for. ohanginetho frequency of; ooqillaqous'l. applied therotoby o predetermined. amount and 'at leest: Que of the. networks. inc :ludingpal -delayrdeviqex the-arrangement being such thatv thel output frornlaa.y first rof ,the networks is delayedf ,relatively to the..o.utpi1t. ,from the second .Uby the time n and .the durations'. of'. the buis eiuasllch that/each burst appearingI at thesoutputy oft ei r'stq network.. coincides at least partly With-the next Succeeding roooiyediburst appearing; at thooutnut oaths Seqqudfnetworin.JV lfthe systeinisgfortransmitting :parle and SPlQe signals. the bursts of' oscillations: urofof two? different.- frequencies alterriatoly and their. frequency; diierence; is constant. Byincluding;thefrequenyf changerFJ device in one ot the: networks..` howoyenl theY bursts o oscillation appearingat theoutputiof the.k outputzcircuit re of, two different frequencies alternately: representative; 0i. the mancano. space-.signant The; froqueucyfchanson Clo-vice maybe .arranged to, either add- .onsubtractt a .lihxedeamount from.. the. oscillations applied theretofrequency changer-moy for .example compriamo-balanced andtheixed amount addedlorl. subtractedvf., ybe-Lof low frequency, Thus. a. low' frequenyr oscillatori may be used in effecting the frequency change; andy as.; is. well knowny a.. low..A frequency oscillatorl cant readilyi be? made stable.. A, superheterodyne receiver,l may. andrusually will` be .ouiriloyelhl butfsdrift in.-.tho. frequency vigilia-local oscillatorA used.. irl the. frequencyrchanger, por-tion o ifrthe superheterodyne receiver affects alll received' signals equally and hence has nov effect on the frequency; of the oscillations. appearingvin the output. of theV receiver.

Accordingtoa feature ofythetinvention thedifferences between. the frequencies of successively-l transmittedl pairs y ofjhursts-are representative of a succession of-litemsof intelligenc.et` Wherebytheirequencies ofthe bursts at., the output. ofthe, receiver are. directly representative; ofthe succession. of items of.. intelligencek respectively.

The invention4 will. nowv be. described, by way of example,T witht reference. tol. the. accompanying; drawings inwhieli,

Figure. 1` a. bloolc schematic. diosratnfofa'. carrier frequency Shift telegraphY systeuii Figures, 2 and... 3. areexplanato-ry..r diagrams:

Figure 4 is a schematic diagram of apparatusat a trausuiittinatorminal-oi 11.1.5. Channel feloprinterrsysfem It'is' convenient to divide Figure 4 into four parts, Figures 4(a) to 4(d), ofwhich Figure 4m) shows an arrangement-of 15 synchronised i automatic senders for generating teleprinter signals in turn,

Figure 4(5) is a diagrammatic sketch of a generator for generating 32 oscillations of different frequencies,

Figure 4(c) is a theoretical circuit diagram of control apparatus for controlling the transmission of oscillations generated by the arrangement of Figure 4(13) in dependence upon signals generated by the automatic senders 'shown in Figure 4(a), and

Figure 4(d) is a block schematic diagram of a transmitter for transmitting the oscillations selected by the arrangement of Figure 4(c).

Figure 5 is a schematic diagram of a receiving terminal for use with the transmitting terminal of Figure 4. lt is convenient to divide Figure 5 into ten parts, Figures 5(a) to 5(h) and 5U) and 5(k), of which Figures 5(a), l(b), (c), (d), (e) and (f) are block schematic diagrams of parts of a receiver for use in receiving signals transmitted by the arrangement of Figure 4, Y

Figures 5(g), (h) and (j) show parts of a distribute for distributing the output of the arrangement shown in Figure 5U) to l5 channels and for providing in each channel voltages of a form suitable for operating a teleprinter, and

Figure 5(k) is a block diagram of a preferred form of synchronising circuit.

Figure 6 is a diagram illustrating the wave form of a teleprinter signal.

Referring to Figure 1, a keying unit 10 is connected to a modulator 11 and serves to apply to the modulator a series of mark and space signals of opposite polarities.

An oscillator 12 is connected to the modulator 11 and aerial 14. It will be assumed that the duration of the mark and space signals are 20 milliseconds or an integral multiple` thereof and that the difference between the frequencies of the transmitted oscillations is 800 c./s.

The transmitted oscillations are received at an aerial v 15 and applied through an R. F. amplifier 16 to a frequency changer 17 where they are mixed with oscillations from a local oscillator 18 to produce beats of intermediate frequency. The intermediate frequency oscillations are amplified in an intermediate frequency amplifier 19 whose output is applied to two transmission channels formed of networks 20 and 21. It will be assumed that the oscillations ofiintermediate frequency are alternately of 1.0 mc./s. and 1.0008 mc./s. respectively representative of the mark and space signals.

The network 20 includes a delay line 22 designed to provide a time delay of 20 milliseconds. The network 21 includes a balanced-modulator 23 to which the oscillations of intermediate frequency are applied as carrier. An oscillator 24 of 1000 c./s. is connected to the modulator 23 to modulate the oscillations of intermediate fre- `the oscillations appearing at the output of the network 20.

The outputs of the two networks are applied to a 'balanced modulator 25 whose output is arranged to be the lower side-band arising from the modulation of the output of the network 21 by the output from the network 20. i

j VIn Figure 2(a) a full line curve 26 represents the `Stb) and (c).

in Figure 3(d).

'is 1,000 c./s.

, 4 i y oscillations of intermediate frequency applied to the two networks 20 and 21 of Figure 1. Each of the intervals t1, t2, t3 and t4 are of 20 milliseconds and it will be seen that the mark and space signals are each of 20 milliseconds. Figure 2(b) represents the output from the network 20 of Figure l and is identical with Figure 2(a) but delayed by 20 milliseconds. Figure 2(c) represents the output from the network 21 and it will be seen from' the curve 26" that the Output from the network 21 is' in phase with the applied oscillations as shown in Figure 2(a) but the oscillations have been modified by the addition of 1000 c./s.

During an interval t1 an oscillation of 1.001 mc./s. is applied to the modulator 25 of Figure 1 as a carrier and is modulated by an oscillation of 1.0008 mc./s. from the network 20. Thus the frequency of the lower side-band is 200 c./s. During the interval t2 an oscillation of 1.0018

'rnc./s. is applied to the modulator 2S from the network 21 and is modulated by an oscillation of 1.0 mc./s. from the network 20. Thus the frequency of the lower sideband during the interval t2 is 1,800 c./s.

Referring to Figure 3, the full line curve 26 of Figure 3(a) represents the input to the two networks 20 and 21, the full line curve 26 of Figure '3(b) represents the resulting output from the network 20, and the full line curve 26 of Figure 3(c) represents the output from the network 21. ln Figure 3 the mark and space signals are of unequal durations but each commences at the beginning of one of the intervals t1 to t5. rFliese intervals are, as in Figure 2, each of 20 milliseconds duration.

It will be seen from Figure 3(a) that the first burst of oscillations applied to the two networks 20 and 21 of. Figure l is of a frequency of 1.0 mc./s. and a duration of 40 milliseconds extending over the intervals t, and t2. This rst burst is delayed by 20 milliseconds in the network 20 and appears as shown in Figure 2(b) at the output of the network 20. The output of the network 21 is as shown in Figure 3(c) and is in phase with the input but increased in frequency by 1000 c./s. The second burst of oscillations applied to the networks 20 and 21 is of a frequency of 1.0008 mc./s. and a duration of 20 milliseconds. The third burst is of the frequency 1.0 mc./s. and occupies the interval t4 of 20 milliseconds and the fourth burst is of the frequency 1.0008 mc./s. and is of 40 milliseconds occupying the intervals t5 and t6.

Referring to Figure 3M), this illustrates the output of the balanced-modulator 2S of Figure l on the application thereto of the bursts of oscillations shown in Figure During the interval r1 the output of the modulator 25 is the difference between the frequencies 1.0008 and 1.001 mc./s. that is to say 200 c./s. as shown During the interval t2 the difference frequency is 1000 c./s., during t3 it is 1,800 c./s., during t4 it is 200 c./s., during t5 it is 1,800 c./s. and during t6 it Referring again to Figure 1, the output of the balancedmodulator 25 is applied to two filters 27 and 28 which are tuned to 200 c./s. and 1,800 c./s. respectively. The outputs of the two filters 27 and 28 are applied through two rectifiers 29 and 30 respectively to the input terminals of a bi-stable multi-vibrator or ip-ilop circuit 31. The output of the circuit 31 is applied to a polarised telegraph relay 32 whose two fixed contacts 33 and 34 are connected to `opposite poles of a lbattery 35 which has a centre tap connected to' earth and the movngcontact 'of the relay 32 is connected to an output terminal 36.

If signals as shown in Figure 2W) are received the 'Koutput of the tbalanced modulator 25 is in the form of bursts of oscillations of equal durations whose frequencies are alternately 200 c./ s. and 1,300 c./s. as previousof the flip-flop circuit changes polarity every 20 milliasse-sev secondsandlthe voltage appearing at the output terminal h'kewise changes polarity every `20 milliseconds whereby the original mark and space signals are reproduced.

If bursts of oscillations as shown in Figure 3.(a) are received during the period t1 the output of the balancedv modulator v25 of"Figure 1 is of the frequency 200 c./s. These-oscillations are ypassedlby the lter 27'and 'the relay 32 becomes energised in one sense. During the interval r11 the output of the balanced-modulator Z5 is of 1000 c./s. and is passed by neither of the 'two lters 27 and 2S and'hence the energisation of the relay 32 remains unchanged. During the interval t3 the output of the balanced-'modulator 25 is of 1,800 c./s. and` hence the energisation of the relay 32 isreversed whereby the polarity of the output at the terminal 36 becomes reversed. During the period@ the output ofthe balanced-modulator 25-is -of 200 c./s. and hencea further `reversal of the output of the relay 32 takes place. During the interval t5 the output of the balancedniodulator 25 is lagain 1,800 c./s. and hence a further reversal of the `output of the relaytakes place and as lthe output during the interval t6 is 1,000 c./s. and is passed by lneither of the two 'filters 27 and 2S the polarity ofthe output during the interval t5 is maintainedduring the interval t5.

Thus the signals appearing at the output of the relay '3'2'are in phase with the bursts as shown in` Figure 3(a) and the original mark and space signals are reproduced. Although an arrangement has -been described in which there is no gap between mark and space signals it will be understood that so long as the duration of each burst is sufficiently long 'to lensure that at least a part of -each 'burstappearing at'the output ofthe network' 20 coincides with the next succeeding burst appearing at the output of the network 21 to produce the appropriate Ioutput from the balanced-modulator 25 satisfactory results will be achieved. v

Furthermore the delay network 22 need not provide a delay of 20 milliseconds as described. Satisfactory results willnot be achieved if the delay timeis longer than 20 milliseconds but the delay time may be made equal to 20 milliseconds divided by an integer.

In yorder to make the band-width of the lters 27 and 2S as small as possible, however, itis necessary to make the 'bursts of oscillations appearing'at the output of the balanced-modulator 25 as long as possible and hence it is preferred to make the time delay equal to 20 milliseconds and` to transmit the Ibursts-without gaps between them.

An application of the invention to the transmission of teleprinter signals willnow be described with reference toFig'ures 4, 5 and 6.

Figure 4(0) shows fifteen automatic senders A1 to A15 of known kind, each having tive peckers (not shown), a pecker bar (not shown) and an operating coil, the tfteen operating coils being shown at L1 to L15 respectively. These senders may each be a Western Uni-on type 22A tape transmitter. The intelligence to be transmitted is in the form of sets` of codedperforations in fteen, iiveunit, standard, perforated tapes, which are passed through the-senders A1 to A15 respectively. The senders are of the notched-on type, that is to say, each time the operating coil of a sender is energized the tape in the sender is moved a predetermined distance (the distance between adjacent sets of coded holes in the tape) by a toothed wheel whose teeth engage perforations in the tape additionalto the coded perforations. The toothed wheel is arranged to rotate through a suitable angle each time the operating coil is energized. Each time the tape comes to rest, one -or more peckers make contact with the pecker bar depending upon the code of the perforationsin the tape.

One terminal of each of the coils L1 to L15 is connected to one terminal of a battery E1 whose other terminal is earthed, and the other terminals of the coils L1 to L15 are connected to fixed contacts 1 to 15 respectively .ot a distributor-D1 whose moving. contact 16` isearthed.

ratio. to a'shaft .S whichy may .bedireetly connectedto. he

Rotation of the ycontact'lti causes the coils 1141041.15, .to'be energised in turnand'hencecauses the tapesin .thezsend'ers A1 to A15 to be notched-on in turn. The moving contact 16 is arrangedto be rotated by a motor (to be described later) at a speed of '360 R. P. M. whereby the senders A1 to A15 Iare each operated'six'times per second.

The pecker bars of the fteen senders are connected tothe'iifteen fixed contacts '1 to 15 respectively of a second distributor D2 whose moving vcontact'16a isearthed. The moving contact of the distributor D2 is mechanically connected to the moving contact `of the vdistributor D1 byany suitable meansshown asa broken lineS 'whereby thetwo moving contacts rotate at the same'speed.- The pecker bars of the senders A1 to A15 are therefore `earthetiiin turn, and each pecker bar is earthed sixvtimes'a second. The moving contact of the distributor D2 is arrangedto lag on that in the distributor' D1 as shown;

Five diode recti'ers R1 to R5 are provided 'foreach of the senders A1 to A15 and the peckers of eachsender are connected to the anodes of the live rectiiiers lrespectively associated therewith. IThe cathodes of the fifteen sets of tive rectiiiers R1 to-R5 are connected 'to terminals T1 to T5 respectively. These terminals areconnected through resistors P1 to P5 respectively to thel negative terminal of a 50-volt battery E2 whose positive terminal is earthed.

The terminals T1 to T5.are normally, therefore, at' a potentialof -50'volts relatively toeanth. Each timefone or'more of the peckers in' the senders :is:y 'earthecl4 through its pecker bar and the distributor D2,.however, the potential ofthe one of the terminals T1 to T5 .to which '.tlie pecker isconnected, is raised tozearthpotential; The pattern of the rpotentials. of the terminals T1 to T5v is, :there fore,` determined at any one :instant by the code of'the perforations in the tapein the sender whose pecker -b'ar is earthed at that instant. This pattern .is :determined vby the senders A1 to A15 .in turn and changes 90 times a second (six per second per sender), 'that .is 'to say each pattern lasts for about 11.11 milliseconds.

It will'be appreciated that the senders `areeach operated at the'standard teleprinter rate of six `characters perfsecond, and it is thought that .15 channels is `the :most that can be operated satisfactorily` in a time'divisiommultiplex system over long distance radio links. It i's well known that reflections bythe ionosphere of a longidis-tancesignal may cause the-duration of a signal to' vary upto three'mil- 'liseconds Signals of durations lessthanabout ten milli- `secondsare liable, therefore, to vbe mutilated to "the-extent of` producing errors at a receiving station. Theli'gure-:of 11.11 .milliseconds per signalprovide's. anv adequate-margin to ensure satisfactory :resultsiunder fnormalionospheric conditions, and allows 15 channels to. be provided.

Referring now to Figure 4(b) the electrostatic generator shown comprises a wheel W which has 32' ring'sof apertures AP formed therein. Thewhee'l W is rotated-by a synchronous motor M1 suppliedwith power at '60c.71s. from a tuning fork TF1. The tuning fork TF1 is` arranged to be maintained in oscillation inl known `manner -at a frequency of 1,800 c./s. to within one part in a million. The tuningfork drive may be type F. K. 2 fork drive made by Times Telephoto Equipment Incorporated. The output from the fork is passed through a 30:1"frequency divider FD and a power amplifier PA to the 'motor M1. A4 series motor SM is f employed: for running nhenwheel up to the synchronous .speedtof' '1=','800"R.1 P; M.,.: and ythe. :shaf of the motor SM is 'coupled` through a `gear "Gibb moving contacts of the distributors D1 andD'gl ofEigure 4(a). i'

Thirty-two collector electrodes (not shown)y vareniranged 'close to the rings of apertures respectively inthe wheel W, and the wheelis given awsuitablefpotential' .by any suitable means (not shown). The number o'f'iper- 'forations in the thirtyatwo rings. is arranged -to'rbe-vsuch thatthe. apertures. in the :thirty-two frings y.passlthe'eolleat'or weer electrodes respectively at the rate of 1,800 per second, 1,890 per second, and so on, in steps of 90 to 4,590 per second. Audio-frequency oscillations of frequencies 1,800 c./s. and in 90 c./s. steps to 4,590 c./s. are therefore made available at the collector electrodes. These electrodes are connected to the thirty-two cores of a thirtytwo core cable CA.

An additional ring of three, equally-spaced apertures AP' is provided in the wheel W for the generation of triggering pulses for la purpose to be described later. A collector electrode (not shown) co-operates with the addi- .Utionhall ringof apertures AP and is connected to a terminal T5. Pulses therefore appear at the terminal T at the rate of one pulse every 11.11 milliseconds. An alternating current generator ACG is connected to the shaft of the Wheel W and provides an output of 90 c./s. for a purpose to be described later. The output of this genyerator is applied to an output terminal Tf1.

Referring now to Figure 4(0) this shows a selector arrangement for selecting oscillations from the arrangement shown in Figure 4(b) in dependence upon the potentials on the terminals T1 to T5 of Figure 4(11). The cable CA and terminal T5 are those shown in Figure 4(1)), and terminals T1 to T5 are those shown in Figure 4(41). The terminals T1 to T5 are connected to the control terminals of five gates G1 to G5 respectively whose input terminals are connected together and to the output terminal of a Kipp relay K2, that is to say a relay which provides an output pulse some time after the application thereto of an operating pulse. The five gates G1 to G5 may conveniently be pentode valves, the terminals T1 to T5 being connected tothe suppressor grids of the five pentodes respectively, and the control grids being connected together and to the Kipp relay K1. The Kipp relay may be a form of one shot multivibrator such as is described in Principles of Radar by the statt of the Massachusetts Institute of Technology, published by McGraw-Hill, chapter 2, article 15.

The outputs of the gates G1 to G5 are applied through delay devices D2', D3' D5 to the input terminals of five cascade-connected flip-flop circuits F1 to F5 respectively, A ip-iiop circuit is a circuit having two input terminals and which can assume either of two stable conditions on the application of a pulse to one or the other or both of the two input terminals and a common form of flip-hop circuit is one in which two triode valves are provided, the anode of each being connected to the control grid of the other through direct current paths each including a resistor and a capacitor connected in parallel.

,One stable condition is that in'which one of the valves is y non-conducting and the other to become conducting. The

two input terminals of the flip-flip circuit may therefore be connected to the control grids of the two valves respectively, and two output terminals may be connected to the anodes of the two valves respectively.

The pairs of output terminals of the flip-dop circuits F1 .to F5 are connectedv through buffer stages B5 to B15 respectively to a rectifier matrix GM. One output termif nal of each of the ip-op circuits, except F5, is connected vto the input circuit of the next to provide a cascade connection. The delay devices D2 to D5 delay the pulses fed therethrough in such a manner that pulses appearing -simultaneously at the outputs of the gates G1 to G5 are applied in succession to the flip-dop circuits F1 to F5, a

pulse-from the gate G2 being applied after the application f of :a pulse from the gate G1, a pulse from the gate G3 1 being appliedaf/terthe application of a pulse from the gate t G1 and so on. The delay between the application of one pulse and the next in the`succession is made such that any cascade action arising from the application of a pulse in the succession is completed before the application of the next pulse in the succession. p

The action of the arrangement of Figure 4(c) as so far described may be as follows:

It will be assumed that all the flip-flop circuits F1 to F5 are initially in their switched-olf condition. The Kipp relay K2 is arranged to provide an output pulse a'few microseconds after the application of a pulse at the terminal T5. The output pulse from the Kipp relay is of a duration of only a few microseconds. This pulse is applied to the gates G1 to G5 and is reproduced at the outputsA of the gates whose control terminals are connected to those of the terminals T1 to T 5 which are at earth potential. The remaining gates provide zero output. Those of the five flip-flop circuits F1 to F5 connected to the gates which pass the pulse from the Kipp relay become switched on. The tive flip-flop circuits are, therefore, set up in conditions determined by the binary code of the set of perforations in the tape engaged by the Y peckers of the one of the senders A1 to A15 [Figure 4(a)] whose pecker bar is earthed at that instant through the distributor D2.

During the next interval of 11.11 milliseconds the Kipp relay provides a further output pulse and a further set of pulses is applied to the terminals T1 to T5 from a difference one of the senders.

Thus the pulse applied to the gates from the Kipp relay is passed by those of the gates whose control terminals are connected to those of the terminals T1 to T5 which are at earth potential.

If a pulse is applied from one of the gates to its corresponding dip-flop circuit and that circuit is already switched-on as a result of the application thereto of a previous pulse this flip-flop circuit becomes switchedot, and delivers an output pulse to the next dip-flop circuit in the cascade F1 to F5. If on the other hand the fiip-lop circuit is "switched-ott the application of the pulse thereto causes the ilip-op circuit to be switchedon.

Thus the five flip-flop circuits are caused to assume conditions related to thediference between the successive pairs of sets of pulses applied to the terminals T1 to T5.

Out of every 11.11 milliseconds (the time between successive pulses applied to the terminal T5 and the time between successive movements of the tapes in the iifteen senders A1 to A15 of Figure 401)) only a few microseconds are used to set up the flip-dop circuits F1 to F5. This allows considerable latitude in the resetting (notchingon) of the tapes in the senders A1 to A15 [Figure 4(a)],

in the movement of the peckers during the notching-on periods, and in the positions of the moving contacts 16 in the distributors D1 and D2.

The two output terminals of the five ip-op circuits are connected to the input terminals of tive push-pull buier stages B5 to B111 respectively, and the outputs of the ve buffers B5 to B10 are applied to the ve pairs of input terminals of a matrix GM of germanium rectiers. A suitable rectifier matrix is described in Proc. I. R. E., February 1949, page 139, under the title Rectifier Networks for Multi-Position Switching, by D. R. Brown. The matrix has thirty-two output terminals and it is arranged in known manner that irrespective of the outputs of thel tive buer stages B5 to B15 as determined bythe settings of the tive ilip-op circuits F1 to F5, only one of the output terminals is at any instant at earth potential, the remainder being at say 10 volts negative relatively to earth. The thirty-two dierent combinations of the settings of the flip-hop circuits F1 to F5 cause the potential of different ones of the thirty-two output terminals of the matrix to be raised to earth potential.

The thirty-two output terminals ot the matrix GM are connected to the control terminals of thirty-two gates respectively, shownA asablock G5. lOnly one of these gates is open, therefore, at any one instant. The corestofthe thirty-two core cableCA are connected to the input' terminals of the thirty-two gates G6 respectively an'd it is arranged that the thirty-two gates 4have a common output terminal T8. The frequency of the oscillation at the terminal T3 at any instant is deterrnined'by the difference between the code of theperforations in the tape in that one of the senders A1 to A15 which has its pecker bar earthed at that instant through the distributor D2 of Figure 4(a) and the next 'preceding code.

lt is arranged that the frequencies of the oscillations transmitted correspond in a particular manner to the 32 five-digit `binary codes setup in the dip-Hop circuits F1 to F5. When F1 is switched on and the' other four are switched off, corresponding to code 1 an oscillation of frequency 2970 c./s. istransmitted. When F2 is switched on and the other four switched off, correponding to code 2, an oscillation of. frequency 3060 c./s. is transmitted, and so on through the co'des in steps of 90 c./s. up to code 19 when' the' highest frequency, 4590 c./s., is transmitted. The next codev 20, F3, F5 on and F1, F2, F4 oif, causes an oscillation' of frequency 1800 c./s. to be transmitted and so on through the codes 20 to 31, in steps of 90 c./s. up to the code 31 when all ve flipflop circuits are switched on and an oscillation of frequency 2790 c./s. is transmitted, and 'finally to code 32, when all flip-flop circuits are switched oi and an oscillation of frequency 2880 c./s. is transmitted. It will be seen that the thirty-two oscillations generated by the arrangement cf Figure 4(b) are all harmonically related, and the purpose of this is to facilitate the. suppression of transients when the frequency of the oscillations at the terminal T8 makes an abrupt change. By suitable selection of the instants Ol' occurrenceof the pulses at the terminal T6, lor by suitable adjustment of the variable delay device VAD associated with the Kipp relay K2, it can be arranged Y that the abrupt changes in frequency at the terminal T8 occur when the oscillations are passing through zero.

Referring to Figure 4(d) this shows a limiter L follcwed by four flip-flop circuits F6 to F9 connected in cascade. The fdp-dop circuits F6 to F9 are arranged in known manner to act as frequency dividerseach having a division ratio of 2:1. Suitable frequency dividers are described in Principles of Radar by the staff of the Massachusetts Institute of Technology,` published by Mc- Graw-Hiil, chapter 2, article 14. An output is taken from each of the flip-flop circuits F5 to Fgand applied to the four mixed contacts respectively of a selector switch SW1. The moving contact of the-selector switch SW1 is connected to a modulator MD to which is alsofed the 90 c./s. `oscillation at the terminal Tf1. The oscillationsapplied to the modulator from the switch. SW1 are amplitude-modulated thereforeatQOc/s. andL itis arranged that the vdepth of modulation isabout 50%. The. purpose in providing this modulation is to facilitate synchronisation at a receiver as will be described later. The output of the modulator is passed through acathodefollower stage CF1 and a transformer N1 to a low-pass filter Z having a cut-off frequency of 5,000 c./s. The

i output of the low-pass filter-is applied to a transmitter TR by means of a suitable transmission line. At the transmitter'the 90 c./s. modulation is removed and the 'demodulated tones are applied to produce frequency The frequency of mitter TR are the same as those appearing at the'terminai' T8.

Receiving equipment will nowbe described which demodulates signals received from the transmitter TR of Figure 4(d), measures the frequency of each 11.1-1 milliseconds burst of oscillations received to decide which it is of the thirtytwo"possible-frequencies, establishes va 'five-unit code corresponding to thata't the transmitting terminal causing oscillations 'of the identiedfrequency to' be transmitted, distributes'the' five-unit codes'to fte'een channels, and'produces' voltages in each 'channel of tl-ie form necessary for operating a standard teleprinter.

Each'burst of oscillations'lasts for ll.l`l milliseconds andv as previously' explained, the i'onosphere may cause changes in the duration of each signal'up'to'about 3;'m'i'lliseconds. An arrangement will be described whereby the first and last quarter of each signal4- is disregardedand measurements are made on thecentre portion, '5 .55..'milliseconds duration, of each burst of oscillations.

Assuming first of all that all five of .the flip-flop circuits F1 to F5 of'Fig. 4'(c')are in the switched-ott condition. `Jt/hen pulses are `applied-through gates G1, G3 and G5 corresponding to code 2l, flip-flop circuits F1, 'F3 and F5 are switched on thereby and the signal of frequcncy 1'890 c./s. is transmitted' to the receiver. When the next teleprinter signal corresponding to, say, code 10, causes pulses to be applied to the flip-flop circuits Fzand F4, these two flip-hop circuits become switched .on where- `circuit F1. Thisfiptiop circuit is switchedl .oit .thereby and by itscascade action causes F210 .switch.ot,.vwhich in'Y turn causes F3 to switch off andso. onuntilall are switchedoff.` A signal of-frequencyzZ'SS'O -c./s. is then transmitted. The frequencyl of this signalv is,.as. before, not directly related to the teleprinter character but such that the difference between the frequency of the transmitted signalI 2880 c./s. and that ofthe last transmitted signal (.2790.c./s.) is representative of. thev character.

Figure 5.(a) 'is aischematic diagram offpar-tof .a suitable1receiver. The signals-are received atan aerial AE and pass by way of an R. F. amplifier RFA. and a frequency changer FC'to van-intermediate frequency ampliiier IFAin accordance with standard superheterodyne receiver-technique. The .output of. the `amplifier IFA-fis passed into two networks, one comprising a delayV line and'the other a balanced modulator BM:1 to which an 'oscillator LO istalso connected.v 1

Thede'lay network DLprovidesadelay of 111.11 milliseconds, that is to say, a delay equal. to the Vduration of eachmeceived signal. A4 signal'appears, therefore, at the outputofthe delay-network DLatthe same time as the next succeeding signal appears at the output ofi the amplifier IFA. The' local oscillator LO is of 5'760c.'/s. As. is well. known, drift in'the frequencyl ofaudioJrequency-oscillators can be madenegligibly small and'hencc ther frequencyy of theoscillator LO can bev assumed 'to be 576`0-c./s. at all times. The balanced'modulatorBM1 serves'to add v5760 c.'/s.t@o the frequency of eachsignal "applied thereto from the amplifier IFA.

The outputs from the-delay network DL and the balanced modulator are heterodyned oneagains't the other vin'asecond` balanced modulator BM2 andthe difference assauts? be any suitable variable attenuator. The signals are applied from the input control-circuit lC1 to an amplifier A MP1 and thence to two phase-changing circuits PC1 and PC2. These circuits provide two outputs whose phases differ by 90 over a wide frequency range. Examples are given in an article by R. B. Dome in Electronics, December 1946, page 112. The phase shift effected by the circuit PC1 is made +45 and that effected by the circuit PC2 is made 45 irrespective of frequency. The. outputs of the two phase-changing circuits PC1 and PC2 are applied to a pulse-forming circuit PF1 which operates in known manner to provide two trains of pulses at twice the frequency of the oscillations applied thereto displaced 90 relatively to one another. This pulse forming circuit provides two differentiated pulses of like polarity from a single cycle of an input oscillation and may operate by limiting to a square wave form in a push-pull circuit and feeding the push-pull outputs from the anodes of the push-pull circuit through diodes into a differentiating network. In this way one positive (or negative) pulse is obtained for each crossing of the mean potential in the input wave-form. A further pulse forming circuit PF1 is provided for a purpose hereinafter to be described. For convenience the train of pulses at +45 will be referred to as the working pulses and the train of pulses at 45 will be referred to as the timing pulses. The working pulses are applied through a cathode follower stage CP2 to a terminal T11, and the timing pulses are passed through a switch SW3 and a cathode follower stage CF3 to a terminal T12. A connection is made from the secondary winding of the transformer N2 to a terminal T13 for a purpose to be described later.

Referring to Figure 5 (c) a tuning fork TF2 is kept in oscillation in known manner at a frequency of 1,80() c./s. to within one part in a million. Electrical oscillations at 1,800 c./s. derived from the tuning fork are passed to a synchronising circuit SC to be described later. The synchronising circuit produces a pulse from each halfcycle of the 1,800 c./s. oscillation and hence the output of the synchronising circuit is in the form of pulses having a recurrence frequency of 3,600 pulses per second. These are passed through two flip-flop circuits F111 and F11 connected in cascade and arranged in known manner (for example as already described) to function as frequency dividers each having a division ratio of 2:1. The output of the flip-flop circuit F11 is therefore in the form of pulses having a recurrence frequency of 900 pulses per second.

The output of the flip-flop circuit F11 is applied to a counter DR. This is in the form of what is commonly known as a decade ring and comprises vten ilip'iiop cir cuits, numbered l to in the drawing, connected in such a manner that at one instant No. 1 flip-flop circuit is in one condition (when in this condition a Hip-flop circuit will be said to be switched on) and hip-iop circuits No. 9 flip-flop`circuit through a buffer stage B11 to an output terminal T17, for purposes to be described later. It is arranged that each time the hip-flop circuits Nos. 1, 7, 8 and 9 are turned od they transmit a pulse to their respective output terminals T12 to T12. Outputs are also taken from tlip-op circuits Nos. 1, 3, 5 and 8 through buffer stages B15 to B12 respectively to an output terminal T111, for a purpose to he described later.

Referring to Figure 5 (d) the terminal T12 is that shown in Figure 5(c) and is connected to one input terminal of a ip-tlop circuit F12. The terminal T12 is ,that shown in Figure 5(b) to which the timing pulses are applied, and is connected to the input terminal of a gate G2. The out- 1 put terminal of the gate G7 is connected to the other input terminal of the flip-flop circuit F12. Each time a pulse appears at the terminal T11, that is to say every 11.11 milliseconds, the flip-hop circuit F12 is switched on. Whenever the circuit F12 is switched on it applies a voltage to the control terminal of the gate G2 which then opens. The next timing pulse appearing at the terminal T12 after the gate G2 is opened passes to the flip-flop circuit F12 and switches it off. Each time the circuit F12 is switched off output voltage therefrom serves to close the gate G7 and to switch on a further ip-ilop circuit F12.

This flip-flop circuit when switched on applies voltage through a cathode follower GF4 to an output terminal T12 and to the input terminal T211 of a timing device CLC to be described later. The timing device CLC is arranged to provide a pulse at its output terminal T21, 5.55 milli seconds after the pulse applied to the input terminal T20. The output pulse at the terminal T21 is applied to the flip-flop circuit F12 to switch it off. As the pulses applied to the terminal '1`1.z occur every 11.11 milliseconds, the

Nos. 2 to l0 are in the other condition (when in this condition a flip-flop circuit will be said to be switched off). An example of a suitable decade ring is given by V. H. Regener in Review of Scientific Instruments, 1946, volume 17, page 185. The first pulse applied to the counter DR switches off No. 1 flip-flop circuit andswitches on No. 2 iiip-op circuit. The next pulse switches off No. 2 nip-flop circuit and switches on No. 3 and so on until ten pulses have been applied, the tenth pulse switching on No. 1. and switching off No. 10. This cycle of opera-tions is repeated for every ten pulses applied and as the pulses have a frequency of 900 pulses per second there are 90 complete cycles of operation every second, or one every 11.11 milliseconds. l

An output is taken from No. 1 flip-diep circuit in the counter DR through a buffer stage B11 to a terminal T11, an output is taken from No. 7 flip-flop circuit through a buffer stage B12 to an output terminal T15, an output is taken from No. 8 ip-op circuit through a buffer stage B111 to an output terminui T16 and an output is taken from voltage at the terminal T12 is in the form of an oscilla tion of square wave form, each half-cycle having a duration of 5.55 milliseconds and commencing at thc instant of occurrence of a timing pulse applied to the terminal T12. It will be remembered that the timing pulses lag 90 on the working pulses applied to the terminal T11 of Figure 5 and hence each half-cycle of the oscillation of square wave form at the terminal T12 commences midway between two working pulses.

Figure 5(e) shows in more detail the arrangement of the timing device CLC of Figure 5(d). A crystalcontrolled oscillator CCO of a frequency of 92.16 kc./s. is connected to a ilip-op circuit F11 which is arranged to derive pulses at a recurrence frequency of 92.160 pulses per second from the oscillations of 92.16 kc./s. These pulses are applied to a gate G2 whose control terminal is the terminal T20 shown in Figure 5(d). Each time the gate G8 is opened by voltage applied to the terminal`T20, the pulses applied to the gate from the hip-flop circuit F11 pass through the gate G2 into a chain of Hip-hop circuits F15 to F 23 each arranged as a frequency-divider having a division ratio of 2:1. The overall division ratio of the chain is, therefore, 512:1 whereby the output of the last divider F23 in the chain is at a frequency of 180 c./s. and has a period of 5.55 milliseconds. The output of the last divider F23 is passed through a pulse forming circuit PF2 to the terminal T21 which is that shown in Figure 5(d). The iirst pulse appearing at the terminal T21 occurs at an instant 5.55 milliseconds after the gate G11 is opened, to within one part in 92,160. In this way the time interval of 5.55 milliseconds is measured with a degree of accuracy sufficient for the purposes of the present embodiment.

Referring to Figure 5U), a gate G2 has its input terminal connected to the terminal T11 which is that shown in Figure 5(b) and hence the working pulses are applied to the input of the gate G2. The control terminal of the gate G9 is connected to the terminal T111 which is that shown in Figure 5(d). The oscillation of square wave form applied to the terminal T12 serves, therefore,v to open and close the gate G2 alternately for periods of 5.55 milliseconds respectively. It will be nosas-s? any ofthe thirty-two possible frequencies ranging' from 5940- pulses per second to 17,100 pulsespersecond.

Irrespective, therefore, of the frequency of the pulses applied to the gate G9 during each intervalI of 5.55'

milliseconds when the gate G9 is open, a whole number ofpulses are applied to the gate from the terminal T11. It will also be remembered that the timing pulses applied to the gate G7 [Figure 2(0)] are delayed 90 on the working pulses. Each cycle of the square wave oscillationapplied to the terminal T19 commences, therefore, between two-working pulses. The gate G9 always opens, therefore, between two working pulses and closes betweentwo working pulses provided the pulses are not affected by noise. In this way possible errors in operation due to the gate G9 opening and closing during working pulses, are voided.

The synchronising circuit SC of Figure ;(c) serves, in a` manner to be described later, to ensure that the gateI G9 opens about 2.77 milliseconds after the commencement of each burst of received oscillations applied tol the-terminals T2 and T11, [Figure 5 (b)], whereby the rst and last quarter of each received signal are prevented from passing through the gate- G9, and only the centreportion of 5.55 milliseconds passes through the gate G9.

Duringv each 5.55 milliseconds when the gateA G9. is open, working pulsesl pass through. the gate G9. i-nto a tive-digit binary counter having tive flip-flop circuits F24 to F22 arranged in known manner. These are two-state ip-flop circuits arranged in cascade so that one of the circuits is operated from a preceding circuit.. An example is given in Electronics, September 19,48, on page lll. At the beginning of each count the five flipfl'op circuits. F24 to F28 are in the switched-oft conditionA as will be described later. It is arrangedy that the rst pulse applied to the counter switches on theip-ilop circuit F24, the next pulse switches. off F24 and: switches on. F25, the third pulse leaves F25 .switched on and switches on F24, the fourth pulse switches off F24 and F25 and switches on F26 and so on. In this way When thek ve ipfilop circuits are switched on they indicate counts of l, 2, 4, 8 and 16 respectively.

The pulses applied to the binary counter may. be at anyY of the thirty-two frequencies ranging from 5,940 pulses per second to 17,100 pulses per second. The number of pulses passed into the binary counter during each. operative period of 5.55 millisecondsmay, therefore, be any of a series between 33 and.95. The application of that one of the series containing 64 leaves the ip-op circuits F24 to F28 all switchedo butY the remainder leave one or more of thedip-flop, circuits switched on, dierent ones, or different combinations, of the flip-op circuits being left switched on after theA application of groups of different numbers` of-*pulses in the series (excluding 32). It isarranged that the conditions of the ve flip-flop circuits F24 to F29after.` the 1 application of a burst of pulses thereto corresponds to terminal of the No. 1 nip-flop `circuit in the decade ring counter DR. About 1.11 milliseconds after the end of each 5.55 milliseconds period when the gate G2 is open a pulse appears at the terminal T11-l which is con- Ytnected to an output terminal of the No. 7 liiip-iiopv circuit in the decade ring counter DR [Figure 5(c)l. This pulse is applied to switch on a flip'op circuitgFzgfFigure 5(f) whose outputis applied through a cathode-follower @F5 to-al common control terminal' of iive gates G19'to G14. The outputs of the live flip-flop -circuits F24 to F22 in the binary counter are appliedgto the input terminals of the tive gates G10 to G14 respectively, and the outputs of the gates G12 to G14y are applied through live cathodefollowers GF6 to CF19'respectively-to iive outputterrninals T22 to T26. Each time thegates G19 to G14 are opened bythe flip-flop circuit F29,the'terminals T22 to T26 assume potentials dependent upon the conditions of the ve iiipllop circuits F24 to F28 respectively in the binary counter.

About 1.11 milliseconds after theygates G19 to G11 are opened a pulse appears at terminal T12.v This pulse is applied to switch off the iiip-flop circuit F29 which in turn closes the gates G12 to G14 and-switches o n a pdiop circuit F29. This flip-flop circuit then applies a resetting voltage to the tive hip-flop circuits F24 to F22 which are all switched loff thereby. About 1.11 milliseconds later a pulse appears at the terminal T17 and is applied to switch off the Hip-flop circuit F211 whereby'the binary counter is left in a condition readyto starty the next count. kAbout 2.22 milliseconds later a pulse appears at the terminal T14 [Figures 5(0) and (d)] and the lastv described cycle of operations starts again.

An arrangement'will nowbe described for distributing the voltages appearing at the terminals T22 to T26 channels and for converting these voltages inV each channel into a series of pulses of the standard form for operatingpa teleprinter. In standard teleprin-ter'practice each transmitted character is represented by'a series of signals the series being of 166 milliseconds duration. Each series consists of a start signal lasting about 22 milliseconds during whichl no current is passed into the vreceiving teleprinter. This signal is followed by tive further signals each lasting about 22' milliseconds during which a currentA of liXed magnitude, or zero current, is passed into the teleprinter dependentupon the characterbeing transmitted. These tive signals are followed by a stop signal lasting about 33 millseconds during which current of xed magnitude is passed into the teleprinter. 'An example of such a series is shown in Figure 6 by a curve TPS. In Figure 6' the ordinate represents magnitude, the abscissa represents time, the intervals z1 tot6 are each of about 22` milliseconds and the interval tf1 is of 33 milliseconds. The signal commences at the beginning of the interval t1 and ends at the end of the interval tf1. It will be seen-that during the interval t1 (start signal) and intervals t4 and t6 zero current is ilowing, and that during the intervals t2, t2', t5 and t1, a current of amplitude h is owing. The signals t2 to t6 carry the information which determines the character to be printed by the receiving teleprinter. Those of signals t2 to t6 of zero amplitude are usually termed space signals and those of amplitude h are usually termed mark signals. Including the combination having ve space signals and that including five mark signals there are 32 possible combinations of mark and space signals.

Referring again to Figure 5(1), it is arranged that when any ofthe ip-lop circuits F24 to` F211 are switched on and their associated gates' G12 to G14 areopen, their associated terminals T22-to1T26 are at earth potential. It is alsoarranged that when the-gates G19 to G14 are closed, and when any of the dip-flop circuits F24 to F28 are switched Off and their associatedgates are open, their associated terminals T22 to T26.are at negative potential.

- Whenever the gates G19 to G14 are opened the respective output terminals T22 to T29 assumeeither'earth potential ora positiveipotential dependent upon whether the respective flip-Hop circuits F24to F28 are switched on or oit. Including the condition in which all the terminals T22 to T26 are at earth potential, and that in which all the terminals T22. to T26 areat the positive potential, there are thirty-.two possible combinations of:A the potentials at the terminals T22 to T26. Thesecombinations are determined by the received audio oscillations vas previously described, and the distributor about to be described serves to convert these combinations of potentials on the terminals T22 to T25 to signals having the previously described form (of which one example is shown in Figure 6), as well as to distribute the successive combinations of potentials ap pearing at the terminals T22 to T25 to l5 channels.

A part of the distributor is shown in Figure (g), and comprises a ring of conducting segments or contacts C1 to C15 which are insulated from one another. A slip ring SL1 is disposed on one side of the segments C1 to C15 and a second slip ring SL2 is disposed on the oppositeL side. The two slip rings SL1 and SL2 are arranged (in any suitable manner) to have potentials +100 volts and -100 volts respectively relatively to earth. Batteries E2 and E1 are shown for this purpose. Two brushes B121 and BH2 are arranged to make connections between the slip ring SL1 and the contacts C1 to C11J in turn as the brushes are rotated. A third brush BRS is arranged to make a connection between the slip ring SL21 and the contacts C1 to C15 as this brush is rotated. The three brushes BR1, BR2 and BR2 are arranged to be rotated in step by a shaft SH in the direction of the arrow whereby the brush BR3 lags on the brushes BR1 and BR2. The brushes BR1 and BR2 are arranged to be in contact with two adjacent contacts at any instant, these brushes being shown in contact with contacts C1 and C15 respectively in the drawing. At the same instant the brush BRS is made to be in contact with the contact next preceding that in connection with the brush BR2. ln the drawing the brush BR3 is shown to be in contact with the contact C14.

The shaft SH is coupled to a 60 c./s. synchronous motor M2 by any suitable gearing of 5:1 ratio shown in the drawing by a broken line SHI.. The shaft of the motor M2 is arranged to rotate at 1,800 R.` P. M. whereby the brushes BR1, BH2 and BR2 make six complete revolutions every second, that is to say, approximately one every 166 milliseconds. ln this Way each of the brushes remains in contact with each of the contacts C1 to C15 in turn for a period of 11.11 milliseconds.

Alternating current for the motor M2 is derived from the voltage appearing at the terminal T12 which is that shown' in Figure 5(c). This voltage is in the form of pulses occurring at a frequency of 360 pulses per second. These pulses are applied, as shown in Figure 5 (g), to a frequency divider having a division ratio of 6:1 and comprising six dip-flop circuits arranged in the form of a counter CR having a rotational frequency of 69 c./s. Outputs are taken from ilip-llop circuits Nos. 2 and 5 in the counter CR and applied to the input of a push-pull amplifier PPA whose output is applied to the motor M2. The motor M2 is synchronized, therefore, to the motor M1 of Figure 4(b).

For the purpose of further description it is preferred to represent the `contacts C1 to C15 of Figure 5(g) in ex-v tended fashion as shown in Figure 5 (h).

Figure 5(h) shows valve apparatus for supplying suitable voltages to a teleprinter in No. l channel.- Fifteen sets of valve apparatus as shown in Figure 501) are provided, the apparatus for channel No. 2 being shown in Figure 5(1') to be described later.

Referring to Figure 5(h) seven thyratrons GT1 to GT1 are provided. These are of the type having two control grids as shown. In order to strike thyratrons of this type it is necessary to apply positive potential to the anode and both grids. A triode V1 is also provided for triggering the thyratrons GT1 to GT1. The cathode of the valve V1 is connected to earth, the control grid through a resistor P1, to a terminal T21 and through a resistor P1 to Contact C15 of the distributor. The terminal T2, is arranged to have a potential of +100 volts and the anode of the valve V1 is connected `to this terminal through a resistor P2. The inner grids of the thyratrons GT1 to GT2 are connected through resistors P9 to P15 respectively and the common terminal of all these resistorsv is connected through a resistor P16 to the junction of two resistors P11 and P12 connected between a terminal T28 and earth. The terminal T211 is arranged to have a potential of -75 volts whereby the inner grids of the thyratrons GT1 to GT1 are normally negatively biased. The anode of the valve V1 is coupled through a capacitor CP1 to the common terminal of the resistors P2 to P15.

The outer grids of the thyratrons GT1 to GT1, are connected through resistors P19 to P22 to the live terminals T22 to T21,` which are those shown in Figure 5U). The outer grids of the thyratrons GT5 and GT', are connected through resistors P22 and P22 respectively to a terminal T22 which is arranged to have a positive potential whereby the outer grids of thetwo thyratrons GT1, and GT7 are normally positive. The cathodes of the thyratrous are connected together and through one winding of a differential relay REL1 to earth. The anodes of the seven thyratrons GT1 and GT1 are connected through resistors P25 to P21 respectively, and the common terminal of these resistors is connected through the other winding of the differentialrelay REL1 to the terminal T21. The anodes of the thyratrons GT1 to GT1 are also connected through resistors P22 to P22 and rectiers R5 to R12 respectively to. contacts C2, C5, C1, C9, C11, C13 and C11 respectively of the distributor. The relay REL1 has a lxed contact FC1 connected to the terminal T21 at I-lO() volts, and a second fixed contact PC2 connectedto earth. The moving contact MC1 of the relay REL1 is connected to an output terminal T211 for connection to a teleprinter input terminal.

In operation, as the brush BR?, passes over the contact C15 the brushes BR1 and BR2 pass over contacts C2 and C1 respectively. During this interval of 11.11 milliseconds a negative-going pulse of 100 volts is applied from the contact C15 to the control grid of the triode V1 causing the anode current of this valve to be cut off and a large positive-going pulse to be applied to the inner grids of the seven thyratrons GT1 to GT7. The thyratrons GTS to GTq have positive potential on their Outer grids from the terminal T 22 and positive potential on their anodes through the resistors P311 and P31 respectively. These two thyratrons strike. It is arranged that the gates G10 to G11 Figure 5(1) are open for about 1.11 milliseconds and it will be remembered that the terminals T22 to T26 have potentials determined by a signal received in channel No. 1. The anodes of the thyratrons GT1 to GT5 have positive potential applied thereto through the resistors P25 to P29 respectively. Those of the thyratrous GT1 to GT5 Whose outer grids are connected to those of terminals T22 to T26 which are at positive potential,

strike, and the remainder stay non-conducting. The

currents flowing in the two windings of the relay RFL1 are equal and hence, throughout this interval of 11.11 milliseconds, the moving contact MC1 is in contact with the earthed xed contact PC2 and hence a space signal is sent to the terminal T211. The brushes move on and during the next five successive intervals each of about 22 milliseconds duration, the contacts C2, C5, C1, C2 and and C11 are made 100 volts positive in turn by the brushes BR1 and BR2. These ve contacts are connected through the rectiiiers R11 to R111 respectively to the anodes of the five thyratrons GT1 to GT5. A positive-going pulse of 22 milliseconds duration is, therefore, applied to these anodes in turn. The cathode current of each of the thyratrons GT1 to GT5 which is struck is therefore increased whereby the relay REL1 is operated causing a mark signal to be transmitted to the terminal T311. The cathode current of each of the thyratrons GT1 to GT5 which is non-conducting remains unaltered whilst the positive pulse is applied to the anodes thereof, and hence a space signal is sent to the terminal T311. In this way tive mark and/or space signals each of about 22 milliseconds duration follow each other in succession in dependence upon the potentials applied to the terminals 

